The apes’ edge: positional learning in chimpanzees and humans

Abstract

A wide variety of organisms produce actions and signals in particular temporal sequences, including the motor actions recruited during tool-mediated foraging, the arrangement of notes in the songs of birds, whales and gibbons, and the patterning of words in human speech. To accurately reproduce such events, the elements that comprise such sequences must be memorized. Both memory and artificial language learning studies have revealed at least two mechanisms for memorizing sequences, one tracking co-occurrence statistics among items in sequences (i.e., transitional probabilities) and the other one tracking the positions of items in sequences, in particular those of items in sequence-edges. The latter mechanism seems to dominate the encoding of sequences after limited exposure, and to be recruited by a wide array of grammatical phenomena. To assess whether humans differ from other species in their reliance on one mechanism over the other after limited exposure, we presented chimpanzees (Pan troglodytes) and human adults with brief exposure to six items, auditory sequences. Each sequence consisted of three distinct sound types (X, A, B), arranged according to two simple temporal rules: the A item always preceded the B item, and the sequence-edges were always occupied by the X item. In line with previous results with human adults, both species primarily encoded positional information from the sequences; that is, they kept track of the items that occurred in the sequence-edges. In contrast, the sensitivity to co-occurrence statistics was much weaker. Our results suggest that a mechanism to spontaneously encode positional information from sequences is present in both chimpanzees and humans and may represent the default in the absence of training and with brief exposure. As many grammatical regularities exhibit properties of this mechanism, it may be recruited by language and constrain the form that certain grammatical regularities take.

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Acknowledgments

The chimpanzee vocalizations were provided by K. Slocombe and S. Townsend. Funding for this experiment was provided by MBB grants to M. Hauser, A. Endress, E. Versace and S. Carden, as well as additional funds to M. Hauser from the Wenner-Gren Foundation, McDonnell Foundation and gifts from J. Epstein and S. Shuman. We are grateful to L. Pharoah, R. Atencia, K. Brown and the Jane Goodall Institute USA and staff of Tchimpounga Sanctuary for their help and enthusiasm with our research. In particular, we appreciate the hard work of the animal caregivers: J. Maboto, B. Moumbaka, A. Sitou, M. Makaya, B. Bissafi, C. Ngoma, W. Bouity, J. Tchikaya, L. Bibimbou, A. Makosso, C. Boukindi, G. Nzaba, B. Ngoma. We also appreciate permission from the Congolese Ministère de la Recherche Scientifique et de l’Innovation Technique for allowing us to conduct our research in their country. We thank J. Call for helpful comments on an earlier draft of this manuscript. Lastly, we thank Brian Hare for his efforts in coordinating the logistical aspects of the work and for helping to make Tchimpounga a superb research facility.